The present invention relates to a plasma ion source in which an ion acceleration voltage for extracting ions from within a plasma is high, and, more particularly, to extraction lenses in the ion source of this type.
A microwave plasma ion source is disclosed in, for example, U.S. Pat. No. 4,058,748 and Japanese Laid Open Patent Publication 55-93644 ions are extracted from within a plasma with a high extraction voltage.
A microwave plasma ion source of the aforementioned type is used in an ion implanter for implanting ions into semiconductor wafers.
When, in the microwave plasma ion source, the acceleration voltage to be applied to the acceleration electrode set at approximately 50 kV in order to increase the energy of the ion beam, a D.C. discharge of unknown cause across the deceleration electrode and the grounded electrode, so the acceleration voltage could not be supplied with a high voltage of at least 50 kV. This problem makes it impossible to cope with the requirements of ion implanters for increasingly higher acceleration voltages, and any countermeasure is desired. Such problem of discharge in an extraction electrode system arises, not only in the microwave plasma ion source as stated above, but also in other plasma ion sources in common.
It is accordingly an object of the present invention to provide a plasma ion source which is free from the problem described above, that is, which can extract an ion beam of high energy.
According to the present invention, a plasma ion source includes a discharge chamber in which a plasma is produced by plasma generation means, an acceleration electrode disposed adjacent to the discharge chamber to extract ions from the produced plasma, a deceleration electrode disposed adjacent to said acceleration electrode to decelerate the extracted ions, a ground electrode disposed adjacent to said deceleration electrode, a container made of an insulator surrounding the discharge chamber and the respective electrodes, and a shield ring electrode of ground potential disposed in a vicinity of the deceleration electrode and along an inner wall surface of the insulator container in order to prevent any discharge from arising across the deceleration electrode and the ground electrode.
Due to the features of the present invention, unlike the prior art in which the electric discharge begins to arise across the electrodes at the acceleration voltage of 50 kV, it is possible to prevent the interelectrode discharge even with an acceleration voltage of 80 kV. As a result, a plasma ion source capable of extracting an ion beam of high energy can be provided, and an ion implanter of high performance can be realized by employing such plasma ion source.